Method and chemical composition for reclaiming of cured elastomer materials

ABSTRACT

Reclaiming cured elastomer material by mixing the cured elastomer as crumbs or chips with a devulcanization-aiding chemical composition, and devulcanizing the cured material by applying a shear-stress deformation to the mixture of the crumbs or chips with a devulcanization-aiding chemical while performing a mechanical disintegration of the cured elastomer into fine-ground crumbs under controllable temperature not exceeding about 90 degrees C., where the chemical composition includes a first agent promoting scission of sulfide bonds of free radicals formed under the shear-stress deformation, which is selected from amines and sulfides and their derivatives, a second agent providing the pre-set acidity during the process inhibiting recombination of sulfide bonds, which is selected from organic acids and their anhydrides, a third agent contributing to fast stabilization of the free radicals, which is selected from oxidants, and a fourth agent promoting redox reaction, which is selected from oxides of metals with variable valence.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/059,642, which is a U.S. National Phase Application under 35 U.S.C.371 of PCT International Application No. PCT/IL2009/000808, which has aninternational filing date of Aug. 17, 2009, and which claims the benefitof priority from Israel Patent Application No. 193,513, filed Aug. 18,2008, whose disclosure is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the field of rubber recyclingand more particularly, the invention pertains to producing devulcanizedor reclaimed rubber from waste vulcanized rubber.

2. Description of the Related Art

Rubber products, such as automotive tires, are usually manufactured byprocessing (molding, extruding, calandering) and subsequentvulcanization of raw rubber materials. Generally, vulcanization is thechemical treatment of a rubber polymer molecules by cross-linking agentsuch as sulfur. The addition of sulfur and other special componentsprovides desired physical properties of a final rubber product, such asstrength, elasticity, durability etc. Vulcanized rubber is the source ofone of the most serious waste product problems. The benefits achieved byvulcanization present a problem when attempting to recycle vulcanizedrubber.

One of the common known approaches to recycling rubber waste isdevulcanization that is intended to depolymerize rubber molecules orbreak the polysulfide linkages. Devulcanization techniques may includemechanical shear, high-energy radiation, chemical processing,thermo-mechanical processing, etc. In chemical terms, devulcanizationmeans reverting rubber from its thermoset, elastic state back into aplastic, moldable state. This is accomplished by severing the sulfurbonds in the molecular structure. With the proper devulcanizationmethod, a much higher percentage of waste rubber, including old tirescan be re-used.

1. It has long been known that mere fine grinding, without anyadditives, brings about cleavage of sulfur bonds and formation of freeradicals. Some early reclaiming processes e.g. disclosed in DE Pat. No.4,425,049 used fine grinding at roller mill for this purpose. Latergroup of American and Japanese scientists found that this process occurseffectively in biaxial extruder at high temperatures that are disclosedin the following patents and patent application: U.S. Pat. No.65,756,680; U.S. Pat. No. 6,590,042; U.S. Pat. Appl. No. US 2001025060.However, it was proved that such process is limited by adverse reactionof so-called “creeping re-vulcanization”, see B. Adhikari, D. De et al.Reclamation and recycling of waste rubber. Progress in Polymer Science.25, 2000. 909-948 and PCT patent application No. WO0129122. From theabove reasoning it is clear that mechanical destruction only is notcapable to provide y reclaimed rubber materials of appropriate quality.

Some traditional devulcanization methods use exposing cured rubber toelevated temperatures for an extended period of time, e.g. by applyingsuperheated steam as disclosed in PCT patent application No. WO9920380.Also is known applying alkali and softeners as further disclosed in U.S.Pat. No. 4,161,464 and No. 5,798,394. However, this “digesting reclaimprocess” not only severs the sulfur bonds in the polymer matrix, butalso breaks the polymer chains, causing a significant degradation ofphysical properties of the rubber. Due to questionable cost efficiencyand environmental problems, thermal devulcanization is rarely usedtoday.

Another technique uses prior swelling of cured material by solvents(mainly, butanol) in order to facilitate subsequent destruction ofsulfide bonds as disclosed in U.S. Pat. Appl. No. US20020091167 and No.US20030225171. That technique requires removal of solvent and dryingreclaimed material making entire process impractical.

Reclaimed materials based on rubber crumb with binders are also widelyused for manufacturing simple articles (tiles, carpets, floors etc.).Usage of various binders in form of adhesives (such as polyurethane) andpolymer additives (such as PP, EVA etc.) are disclosed in the followingpatents and patent application U.S. Pat. No. 5,397,825; PCT Pat. Appl.No. WO9948960; U.S. Pat. No. 5,303,661 and U.S. Pat. No. 4,378,067. Suchreclaimed materials have limited usage due to poor physical propertiesthereof.

Various oxidizers for stabilization free radicals, formed in grindingprocess, have drawn attention of rubber chemists. Chlorine as oxidizingagent is used in technologies disclosed in U.S. Pat. No. 5,693,714; No.5,506,283 and No. 5,438,078. Due to poor quality of reclaimed materialand polluting effect of chlorination agent, this direction did not gainacceptance among industrialists.

Better physical properties of devulcanizate could be obtained byapplying ozone as oxidizing agent as disclosed in LV patent No. LV13339Bof Zagars. Unfortunately, grinding under ozone stream requires toocomplicated equipment, making this technology impracticable.

According to the following patents: U.S. Pat. No. 5,677,354; No.5,798,394 and No. 5,891,926 devulcanization could be performed byapplying some biotechnological reactions. These methods requirecomplicated equipment, large floor space and excessive operating time.

Initially it was considered that cleavage of sulfide bonds occurs onlyat elevated temperature, but later it has been found that in thepresence of amines and disulfides the devulcanization processaccelerates sharply and even could be preformed at ambient temperature(see Krebs. Z. Anorg. Allg. Chemie. 276, 1954). This discovery hasinitiated a number of devulcanization technologies. One of them isrepresented in the following patent and patent applications: EP Pat.Appl. No. EP0690091 and U.S. Pat. No. 5,770,632 (both relate toso-called De-Link process). Other versions are disclosed in PCT Pat.Appl. No. WO2007062611 and WO0129121 and U.S. Pat. No. 6,924,319. Theprocess gained wide acceptance due to simplicity of used standardequipment (roller mills). However, it was shown that sufficientdisadvantages inhere to this technique. It requires fine grinding thecured rubber (up to 0.4 mm and less). Moreover, this method does notprovide good selective action to different bonds and along with theattack of C—S and S—S bonds, it attacks also C—C bonds and initiatestheir deep destruction. This causes reduction of the physical andmechanical properties of secondary vulcanizate. According to thesepublications, secondary devulcanizate from tire scraps retains 62-70% oftensile strength as compared with primary vulcanizate.

Therefore, a need still exists for a method of devulcanization andchemical composition that provide efficient and simple devulcanizationtechnique.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an economical andsimple process for producing reclaimed rubber with minimal deterioratingthe main properties. The entire previous reasoning brings out clearlythat any technology aimed to high retention of initial properties ofelastomers should not be restricted to mere mechanical disintegration.Any effective technology should contain chemical agents reducing thenegative influence of related factors.

According to one broad aspect of the present invention, proposed is amethod of reclaiming of a cured elastomer material, comprising the stepsof:

a) mixing the cured elastomer in form of crumb or chips with adevulcanization-aiding chemical composition,

b) applying a shear-stress deformation to said mixture;

wherein said chemical composition comprises ingredients as follows:

first agent promoting scission of sulfide bonds of free radicals formedunder the shear-stress deformation, which is selected from the followingclasses: of amines and sulfides and their derivatives;

(ii) second agent providing the pre-set acidity in the course of theprocess and thus inhibiting recombination of sulfide bonds, which isselected from the following classes: organic acids and their anhydrides;

(iii) third agent contributing to fast stabilization of the freeradicals, which is selected from the class of oxidants;

(iv) fourth agent promoting redox reaction, which is selected from saltsof metals with variable valence; and

(v) fifth agent prevention agglomeration of said elastomer materialafter applying the shear-stress deformation and reduction of friction,which is selected from the class of polar sorbents.

Preferably, a weight percent ratio of said agents ranges as follows:

First agent—from about 20 to about 75%;

Second agent—from about 15 to about 70%;

Third agent—from about 3 to about 15%;

Fourth agent—from about 2 to about 10%;

Fifth agent—from about 0 to about 5%.

According to still another aspect of the present invention, applyingshear-stress deformation is applied under controlled temperature,preferably it is ranging from about 40 degree to about 90 degree C.

The ratio of the devulcanization-aiding chemical composition to thecured elastomer preferably is from about 0.015 to about 0.05.

According to another broad aspect of the invention, proposed is adevulcanization-aiding chemical composition for obtaining a curablematerial from cured elastomer material, comprising:

(vi) first agent promoting scission of sulfide bonds of free radicalsformed under a condition of shear-stress deformation, which is selectedfrom the following classes: of amines and sulfides and theirderivatives;

(vii) second agent providing the pre-set acidity in the course ofdevulcanization process and thus inhibiting recombination of sulfidebonds, which is selected from the following classes: organic acids andtheir anhydrides;

(viii) third agent contributing to fast stabilization of the freeradicals, which is selected from the class of oxidants;

(ix) fourth agent promoting redox reaction, which is selected from saltsof metals with variable valence; and

(x) fifth agent preventing agglomeration of said elastomer materialafter applying the shear-stress deformation and reduction of friction,which is selected from the class of polar sorbents.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Present invention based on understanding that technology aimed to highretention of initial properties of elastomers could not be restricted tomere mechanical disintegration or applying some devulcanization chemicalagent only. Present invention based on providing shear-stressdeformation during mechanical destruction of cured elastomer, e.g.rubber in form of rough crumb or chips in presence ofdevulcanization-aiding chemical composition reducing the negativeinfluence of various related factors.

The choice of chemical composition is based on mechanism of rubberdestruction.

Suggested mechanisms of destruction could be illustrated as follows:

a) Rupture of the main chain to yield radicals.

b) Fragments of main chain stabilize by taking on oxygen from theoxidants and/or from the air to the points of rupture.—CH₂—C(CH₃)═CH—CH₂.+‘O.’→O—CH₂—C(CH₃)═CH—CH₂—

c) Destruction of polysulfide bonds proceeds by reaction.

Radicals formed at reaction of disintegration are unstable, and hencereactions of stabilization proceed simultaneously. It was found thatprocess of rupture of sulfide bonds is reversible. Recombination of thesulfide bonds occurs in the absence of preventing chemical agents by twoalternative ways:

Vacant bonds may connect into the cyclic structure:

Recombination results in the so-called “creeping re-vulcanization”.

Action of sulfide and disulfide derivatives as well as amine derivativesis explained by formation of intermediate compound with the polysulfidechain:

This reaction provides covering of free bonds and prevention fromrecombination.

It was found out by the inventor of the present invention that equimolarmixture of sulfide and amine derivatives is acting more effectively thanindividual sulfide or amine separately. This effect is presumably causedby formation of complex compound. To this end, equimolar mixture ofsulfide and amine derivatives could be preferably chosen as component,which promotes scission of sulfide bonds of the radicals during finegrinding.

In accordance with general aspect of the present inventiondevulcanization-aiding chemical composition provides the followingeffects:

1. Promotion of scission of sulfide bonds of the radicals during finegrinding at substantially ambient temperatures.

It could be provided by incorporation substances, which promote scissionof sulfide bonds of the radicals during fine grinding. These substancescould be chosen from classes of sulfides and amines and theirderivatives. Usage of complex compounds of sulfides and amines (adducts)in equimolar ratios are preferred.

2. Inhibiting recombination of sulfide bonds and preventing the adversereaction of re-vulcanization.

It could be provided by incorporation substances, which are chosen fromclasses of organic acids and their anhydrides.

Amount of acid could be defined by gradual addition of acid todevulcanization-aiding chemical composition until pH value of materialwater extract reach the preset value.

It could be done by using the following technique: adding 100 ml. ofwarm water to 10 gr. of mixture of rubber crumb with chemicalcomposition, mixing thoroughly and keeping for 2 hours. Further, acidityof poured off water is measured by pH-meter or by litmus paper.Preferably pH of water extract should not exceed value of 0.6

Periodic measurement and thorough monitoring of pH during ofdevulcanization could be performed.

Preferably, dibasic organic acids could be used, since less amount ofthereof is required in order to provide the same acidity.

Taking into account mechanism of reactions occurring in this process,hydroxy radical —OH is favorable to scission of sulfide bonds. To thisend it is preferably to use hydroxy acids.

Thus, dibasic hydroxy organic acids, such as tartaric, malic(hydrosuccinic) acids are the most preferable.

2. Stabilization of free radicals formed in the course of comminuting bytheir oxidation. This effect could be provided by incorporation ofsubstances, which are chosen from oxidants disposed to easy splittingoff atomic oxygen at the temperatures of comminuting (40-80° C.).Oxidants such as zinc peroxide, benzoyl peroxide, dicumyl peroxidedirectly oxidize free bonds of the radicals.

3. Catalysis of the redox reaction.

This effect could be provided by incorporation of substances, which arechosen from substances, which catalyze redox reaction and thus aid tostabilization of free radicals at ambient temperatures. While oxidantsdirectly oxidize free bonds of the radicals, hydroxy acids—for example,tartaric acid—enter into exchange reaction with radicals. Oxides orsalts of metals with variable valence—iron oxides, manganese (IV) oxideand similar act as catalysts of oxidation process and serve to thistarget best.

4. Prevention of agglomeration of particles and reduction of friction.

This effect could be provided by incorporation of substances,facilitating to fine disintegration of rubber vulcanizate and preventingagglomeration of fine-ground rubber crumb. The less is particle size,the more its surface is developed and the more distinct isagglomeration. Agglomerated powder loses its granular properties andconsists from flimsy blocks. Polar sorbents, capable to dispersion inrubber, suit to this destination best. Besides, polar sorbents arecapable for adsorption of accelerators at temperatures of processing andrelease them at temperature of vulcanization. By this way, they do notaffect properties of rubber compounds. Preferably, zeolites withparticles size 1-5 mkm could bee used as sorbents.

It should be noted that two or possibly even more above-mentioned effectcould be provided by one component of chemical composition. For exampleFe₃O₄ could provide both catalysis of the redox reaction and preventionagglomeration of particles and reduction of friction. Preferably, Fe₃O₄produced for using as a bio-pigment with particles size 30-40 mkm, andcommercially available from Israeli company BIOPIGMENT could be used forthose purposes.

Preferably, a weight percent ratio of said said agents ranges asfollows:

First agent—from about 20 to about 75%;

Second agent—from about 15 to about 70%;

Third agent—from about 3 to about 15%;

Fourth agent—from about 2 to about 10%;

Fifth agent—from about 0 to about 5%.

Devulcanization-aiding chemical composition could be prepared beforeprocessing and mixed with rubber crumb or chips using common knowntechniques. Otherwise, ingredients of chemical composition could bemixed directly with rubber to be processed.

Preferably, weight percent ratio of Devulcanization-aiding chemicalcomposition to rubber to be processed ranges from about 0.015 to about0.05.

Devulcanization process in accordance with present invention preferablyperformed under temperature regimes closed to substantially ambienttemperatures, e.g. 40-90° C.

In case of exceeding temperatures applying cooling, e.g. water based isdesired. If temperature is not exceed 120° C., it is possible not toapply cooling.

In order to understand the invention and to see how it may be carriedout in practice, preferred embodiments will now be described, by way ofnon-limiting examples only.

Example 1

Cured rubber crumb from whole tires ground to pieces 3-7 mm in size andseparated from metal and fiber was used as raw material for thisembodiment.

Devulcanization-aiding chemical composition was prepared by formulation:

-   -   Adduct prepared by mixing mercaptobenzothiazodisulphide (MBTS)        with diphenylguanidine (DPG) in equimolar ratio (66.2/33.8        weight percent)—1.4 w.p.    -   Iron oxide Fe₃O₄—0.25 w.p.    -   Zeolite—0.1 w.p.    -   Zinc peroxide ZnO₂—0.15 w.p.    -   Malic (hydroxysuccinic) acid—added until pH of the water extract        of the agent is reached the value 0.3. It was found that in this        example 2.2 w.p. of acid is needed to fulfill this condition.

Chemical agent was mixed with raw rubber material by ratio: rubbercrumb—100 w.p.; chemical agent—4.1 w.p.

The process was carried out by 40 passes of the mixture rubber withagent through the nip 0.2 mm of the standard 2-roll mill at controllabletemperatures 40-90° C.

The process is accomplished at controlled acidity and contents of waterextractable components in the mixture.

Dynamics of variation pH in the process of devulcanization in thepresence of Devulcanization-aiding chemical composition on the base oftartaric acid is given in the table.

TABLE Contents of water extractable Stage of process pH componentsInitial rubber crumb 7.0 0.6% Mixture of crumb with 0.3 3.1% chemicalagent After pass #1 5.2 1.6% After pass #4 5.5 0.9% After pass #7 6.00.6% After pass #10 6.3 0.6% Just after pass #40 6.5 0.6% Devulcanizedmaterial after 5.0 0.6% 2 hours storage

Example 2

Rubber tread buffing pure from metal and fiber was used as raw materialfor this embodiment. Devulcanization-aiding chemical composition wasprepared by formulation:

-   -   Adduct prepared by mixing mercaptobenzothiazol (MBT) with        hexamethylenetetramine (HMTA) in equimolar ratio (53/47 weight        percent)—1.3 w.p.    -   Iron oxide Fe₃O₄—0.3 w.p.    -   Dicumylperoxide—0.2 w.p.    -   Zinc peroxide ZnO₂—0.3 w.p.    -   Tartaric acid—added until pH of the water extract of the agent        is reached the value 0.4. It was found that in this example 1.8        w.p. of acid is needed to fulfill this condition.

Devulcanization-aiding chemical composition was mixed with raw rubbermaterial by ratio: rubber tread buffing—100 w.p.; chemical agent—3.9w.p.

The process was carried out by applying shear-stress deformation duringmechanical disintegration on the mixture of rubber withDevulcanization-aiding chemical composition.

The process is accomplished at controlled acidity of water extract.

Values pH of water extract measured in the beginning was found as 0.4;the same measured in the end of process of devulcanization was found as5.2.

The compound was prepared on base on reclaimed material by formulation:

Reclaimed material—50 w.p.; NR—100 w.p.; Sulfur—3.5 w.p.; ZnO—5 w.p.;Stearic acid—1 w.p.

Samples were cured at temperature 155° C. during 12 min.

Testing has shown properties: Tensile Strength—24.5 MPa; Elongation atbreak—477%.

Example 3

Cured rubber rejects from production of plumbing and sealing fixtureswere used as raw material for this embodiment.

Devulcanization-aiding chemical composition was prepared by formulation:

-   -   Adduct prepared by mixing N-cyclohexyl-2-benzothiazole        sulfenamide (CBS) with hexamethylenetetramine (HMTA) in        equimolar ratio (60.4/39.6 weight percent)—1.4 w.p.    -   Iron oxide Fe₃O₄—0.2 w.p.    -   Dibenzylperoxide—0.2 w.p.    -   Manganese (IV) oxide MnO₂—0.05 w.p.    -   Oxalic acid dihydrate—added until pH of the water extract of the        agent is reached the value 0.5. It was found that in this        example 1.4 w.p. of acid is needed to fulfill this condition.    -   Zeolites—0.3 w.p.

Devulcanization-aiding chemical composition was mixed with raw rubbermaterial by ratio: rubber tread buffing—100 w.p.; chemical agent—2.2w.p.

The process was carried out by applying shear-stress deformation duringmechanical disintegration on the mixture of rubber withdevulcanization-aiding chemical composition.

The process is accomplished at controlled acidity of water extract.

Values pH of ground crumb measured in the beginning was found as 0.3;the same measured in the end of process of devulcanization was found as5.1.

The compound was prepared on base on reclaimed material by formulation:

Reclaimed material—50 w.p.; NR—100 w.p.; Sulfur—2.5 w.p.; ZnO—5 w.p.;Stearic acid—1 w.p.

Samples were cured at temperature 1550 C during 12 min.

Testing has shown properties: Tensile Strength—18.1 MPa; Elongation atbreak—465%.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the embodiments of theinvention as hereinbefore exemplified.

What is claimed is:
 1. A method of reclaiming of a cured elastomermaterial, comprising the steps of: a) mixing said cured elastomer inform of crumbs or chips with a devulcanization-aiding chemicalcomposition, and b) devulcanizing said cured material by applying ashear-stress deformation to said mixture of the crumbs or chips with adevulcanization-aiding chemical while performing a mechanicaldisintegration of the cured elastomer into fine-ground crumbs whereinsaid chemical composition comprises ingredients as follows: (i) a firstagent which is selected from the classes of complex compounds formedfrom amines and sulfides in equimolar ratio; (ii) a second agent whichis selected from the classes of organic acids containing an hydroxyradical; (iii) a third agent selected from the class of oxidants, and(iv) a fourth agent selected from oxides of metals with variablevalence.
 2. A method according to claim 1, wherein a weight percentratio of said agents ranges as follows: said first agent from about 20to about 75%; said second agent from about 15 to about 70%; said thirdagent from about 3 to about 15%; and said fourth agent from about 2 toabout 15%.
 3. A method according to claim 1, wherein ratio of thedevulcanization-aiding chemical composition to said cured elastomer isfrom about 0.015 to about 0.05.
 4. A method according to claim 1,wherein the organic acids of said second agent are mono- and dibasicorganic acids.
 5. A method according to claim 1, wherein the organicacids of said second agent are acids containing hydroxy radical.
 6. Amethod according to claim 1, wherein said third agent are peroxidescharacterized by easily splitting off atomic oxygen.
 7. A methodaccording to claim 1, wherein said variable valence metals are iron andmanganese.
 8. A method according to claim 1, wherein the fourth agentcomprises oxide Fe₃O₄.
 9. A method according to claim 8, wherein saidoxide Fe₃O₄ is used in a form of fine powder.
 10. A method according toclaim 8, wherein said oxide Fe₃O₄ is characterized by particles sizefrom about 30 to about 40 mkm.
 11. A method according to claim 1 whereinsaid chemical composition further comprising a fifth agent selected fromthe class of polar sorbents.
 12. A method according to claim 11 whereinsaid fifth agent comprised with a weight percent ratio from about 0 toabout 5%.
 13. A method according to claim 1 wherein said cured elastomermaterial comprising a range of crumbs or chips sizes of few mm.
 14. Amethod according to claim 13 wherein said cured elastomer materialcomprising a range of crumbs or chips sizes of about 0.5 mm to 7 mm. 15.A devulcanization-aiding chemical composition for obtaining a curablematerial from cured elastomer material by applying a mechanicaldisintegration, said composition comprising at least four agents withfollowing weight percent ratio: a first agent from about 20% to about75% selected from the classes of complex compounds formed from aminesand sulfides in equimolar ratio; a second agent from about 15% to about70% selected from the classes of organic acids containing an hydroxyradical; a third agent from about 3% to about 15% selected from theclass of oxidants; and a fourth agent from about 2% to about 15%selected from oxides of metals with variable valence.
 16. A chemicalcomposition according to claim 15, wherein the organic acids of saidsecond agent are mono- and dibasic organic acids.
 17. A chemicalcomposition according to claim 15, wherein the organic acids of saidagent are acids containing hydroxy radical.
 18. A chemical compositionaccording to claim 15, wherein said fourth agent comprises oxide Fe₃O₄.19. A chemical composition according to claim 15 further comprising afifth agent selected from the class of polar sorbents.
 20. A chemicalcomposition according to claim 19, wherein said fifth agent comprisedwith a weight percent ratio from about 0 to about 5%.